Method for the selective production of ordered carbon nanotubes in a fluidised bed

ABSTRACT

A method for the selective production of ordered carbon nanotubes includes decomposition of a carbon source in the gaseous state in contact with at least one solid catalyst, taking the form of metallic particles borne by carrier grains. The catalyst grains are adapted so as to be able to form a fluidised bed containing between 1% and 5% by weight of metallic particles having average dimensions of between 1 nm and 10 nm. The decomposition takes place in a fluidised bed of catalyst grains. The method can be used to obtain pure nanotubes with predetermined dimensions in a high yield.

[0001] The present invention relates to the production of ordered carbonnanotubes.

[0002] Ordered carbon nanotubes within the meaning of the presentinvention have a tubular structure of diameter between 0.4 nm and 50 nmand a length greater than 100 times their diameter, in particularbetween 1000 and 100,000 times their diameter. They may exist eithercombined with particles of metallic catalyst, or separate from theseparticles. Carbon nanotubes have been described for a long time (S.Iijima “Helical nanotubules of graphitic carbon” Nature 354, 56 (1991)),but are still not exploited on an industrial scale. They couldnevertheless be employed in numerous applications and in particularcould be extremely useful and advantageous in the production ofcomposite materials, flat screens, tips for nuclear power microscopes,storage of hydrogen or other gases, as catalyst supports, etc.

[0003] U.S. Pat. No. 4,663,230 and U.S. Pat. No. 5,500,200 describe aprocess for the catalytic preparation of carbon fibrils by hightemperature decomposition of a source of gaseous carbon in contact witha solid catalyst in the form of metallic particles of size 3.5 nm to 70nm, comprising at least one transition metal, carried by granules ofsolid support of size less than 400 μm. According to these documents thefibrils obtained should comprise an internal core of less ordered carbonsurrounded by an external region of ordered carbon, and should have adiameter varying between 3.5 nm and 70 nm. U.S. Pat. No. 5,500,200discloses that the process for obtaining these fibrils may be carriedout in a fluidised bed, but does not provide any example of such aprocess. All the examples mentioned are carried out with a fixed bed,produce a moderate yield with respect to the carbon source (<20% byweight), and the actual characteristics of the products obtained are notgiven. These documents therefore do not provide any real informationrelating to the production of real nanotubes of ordered carbon and/orthe use of a fluidised bed for the production of such nanotubes.

[0004] Other documents disclose the production of nanotubes ofsingle-wall carbon by means of a catalytic composition formed frommetallic particles that are either carried by support granules depositedin a crucible (WO-0017102) or introduced in the form of an aerosol(WO-9906618) to a reactor fed with a gaseous source of carbon such ascarbon monoxide or ethylene. The yields obtained (nanotubes producedwith respect to the source of carbon) with such processes are very low,and a certain amount of particles of pyrolitic or amorphous carbon isproduced. However, it is important for the practical industrialexploitation of carbon nanotubes to be able to control precisely andsimultaneously the dimensional characteristics, the production yieldsand the purity of the product obtained.

[0005] WO 01/94260, published on 13 Dec. 2001, describes a process andan apparatus for the production of carbon nanotubes in several stages,in which a preliminary treatment stage of the catalyst in order toextract air from the latter is followed by a stage involving thereduction of the catalyst. In such a process it is also necessary toeliminate the amorphous carbon formed in the reaction, which is thus notselective with regard to the nanotubes that are formed.

[0006] U.S. Pat. No. 4,650,657 and U.S. Pat. No. 4,767,737 describe aprocess for the production of a fibrous carbon-containing materialcontaining a ferrous metallic component in a fluidised bed bydecomposition of carbon monoxide in the presence of hydrogen and aneutral gas such as nitrogen, a powder of ferrous metallic catalyst andin the presence of an abrasive such as alumina that acts as a support.These documents mention that the effect of such a fluidised bed is toremove the carbon formed from the surface of the granules, to promotethe fragmentation and to minimise the size of the reactive mass of thefluidised bed. These documents do not describe a process that can beapplied to the production of carbon nanotubes. On the contrary, theproducts obtained are particles of carbon of average dimension 1μ to 50μ(Table 1 of U.S. Pat. No. 4,650,657).

[0007] The publication “Fe-catalyzed carbon nanotubes formation” by K.Hernadi et al., Carbon, 34, No. 10, (1996), 1249-1257 describes aprocess for the production of carbon nanotubes on various catalysts in afixed bed or in a so-called “fluidised bed” reactor of 6.4 mm diameter.Such a diameter cannot produce a true fluidised bed. The catalysts areprepared by impregnation. This process limits the amorphous carbonproduced to a laboratory scale exploitation and teaches that the use ofsuch a “fluidised bed” would be less suitable than the use of a fixedbed.

[0008] In addition FR-2 707 526 describes a process for the preparationof a catalyst by chemical deposition in the vapour phase of metallicparticles of size less than 2 nm in a fluidised bed of porous supportgranules at a temperature of less than 200° C. This document describesmore particularly the preparation of a rhodium-containing catalyst anddoes not describe a catalyst suitable for the production of carbonnanotubes.

[0009] The object of the invention is thus to provide a process for theselective production of true nanotubes of ordered carbon of homogeneousaverage dimensions (varying only slightly around a mean value) underconditions compatible with an industrial-scale exploitation,particularly in terms of yield with respect to the carbon source,catalytic activity and production costs, and of purity in nanotubes ofthe product obtained.

[0010] The invention also provides such a process in which thecharacteristics of the nanotubes produced may be predetermined andadjusted by simple modification of the parameters involved in theimplementation of the process.

[0011] The invention provides more particularly such a process in whichthe yield of produced nanotubes with respect to the carbon source isequal to or greater than 80% by weight.

[0012] The invention also provides a catalytic granular composition thatmay be used in a process for the production of ordered carbon nanotubesaccording to the invention, as well as a process for the preparation ofsuch a catalytic granular composition.

[0013] (Throughout the text, all the terms and criteria relating to thecharacteristics of the fluidised bed are adopted within the meaninggiven in the reference work “Fluidization Engineering”, Kunii, D.;Levenspiel, O.; Butterworth-Heinemann Edition 1991.)

[0014] To this end, the invention relates to a process for the selectiveproduction of ordered carbon nanotubes by decomposition of a source ofcarbon in the gaseous state in contact with at least one solid catalystin the form of metallic particles comprising at least one transitionmetal carried on granules of solid support, these support granulescarrying the metallic particles, so-called catalyst granules, capable ofbeing able to form a fluidised bed, the metallic particles having a meandimension between 1 nm and 10 nm as measured after activation by heatingto 750° C., in which a fluidised bed of the catalyst granules isproduced in a reactor, the so-called growth reactor (30), and the carbonsource is supplied continuously to the said growth reactor (30) incontact with the catalyst granules under conditions suitable forensuring the fluidisation of the bed of catalyst granules, thedecomposition reaction and the formation of nanotubes, characterised inthat:

[0015] the catalyst granules of each catalyst are produced beforehand bydeposition of metallic particles on support granules in a fluidised bedof support granules formed in a reactor, the so-called depositionreactor (20) supplied with at least one precursor capable of forming themetallic particles, and in such a way as to obtain catalyst granulescomprising a proportion by weight of the metallic particles of between1% and 5%,

[0016] the catalyst granules are then placed in the growth reactor (30)without coming into contact with the external atmosphere, which isfollowed by the formation of the fluidised bed of the catalyst granulesand the formation of nanotubes in the growth reactor (30).

[0017] The inventors have surprisingly found that, contrary to theteaching of U.S. Pat. No. 4,650,657 and U.S. Pat. No. 4,767,737, the useof one fluidised bed to produce the catalyst(s) and of another fluidisedbed to produce the nanotubes, without the catalyst(s) coming intocontact with the atmosphere, under the conditions of the invention, notonly does not result in the fragmentation of the carbon-containingproducts growing on the granules, but on the contrary enables orderedcarbon nanotubes of very homogeneous dimensions (varying only slightlyaround the mean value) to be selectively produced, and in a yield ofmore than 80% by weight with respect to the carbon source.

[0018] The catalyst is not subjected to any atmospheric pollution, andin particular is not oxidised between its preparation and its use in thegrowth reactor.

[0019] Advantageously and according to the invention, the depositionreactor and the growth reactor are separate. Advantageously andaccording to the invention, the deposition reactor and the growthreactor are connected by at least one airtight line and the growthreactor is supplied with catalyst granules through this line. As avariant, the granules of the catalyst may be recovered and transferredfrom the deposition reactor under an inert atmosphere. Advantageouslyand according to the invention, the catalyst granules are produced bychemical deposition in the vapour phase.

[0020] According to another possible variant of the invention, one andthe same reactor may be used both as deposition reactor and as growthreactor. In other words, the two stages of preparation of the catalystgranules (deposition) followed by production of the carbon nanotubes(growth) may be carried out successively in one and the same reactor bymodifying the gases and reactants at the inlet of the reactor as well asthe operating parameters between the two stages.

[0021] Advantageously and according to the invention, the fluidised bedof the catalyst granules is formed in a cylindrical growth reactor ofdiameter greater than 2 cm and having a wall height capable ofcontaining 10 to 20 times the volume of the initial non-fluidised bed ofthe catalyst granules as determined in the absence of any gaseous feed.Such a reactor enables a true fluidised bed to be formed.

[0022] Advantageously and according to the invention, a fluidised bed ofthe catalyst granules is formed under a bubbling regime at leastsubstantially free of leakage.

[0023] Furthermore, advantageously and according to the invention, inorder to form the fluidised bed of catalyst granules:

[0024] a bed of catalyst granules is formed at the bottom of the growthreactor,

[0025] the growth reactor is fed underneath the bed of catalyst granuleswith at least one gas whose velocity is greater than the minimalvelocity of fluidisation of the bed of catalyst granules and less thanthe minimal velocity of occurrence of a plunger-type regime.

[0026] Advantageously and according to the invention, in order to formthe fluidised bed of the catalyst granules, the growth reactor is fedunderneath the catalyst granules with the carbon source in the gaseousstate, and with at least one neutral carrier gas.

[0027] More particularly, advantageously and according to the invention,the growth reactor is fed with at least one carbon-containing precursorforming the carbon source, at least one reactive gas, and at least oneneutral gas, which are mixed before being introduced into the growthreactor. The term “reactive gas” is understood to denote a gas such ashydrogen that is capable of participating in and promoting theproduction of nanotubes.

[0028] Advantageously and according to the invention, the source ofcarbon comprises at least one carbon-containing precursor selected fromhydrocarbons. Among the hydrocarbons that may advantageously be used,there may be mentioned ethylene and methane. As a variant or incombination, there may however also be used an oxide of carbon, inparticular carbon monoxide.

[0029] Advantageously and according to the invention, the molar ratio ofthe reactive gas(es) to the carbon-containing precursor(s) is greaterthan 0.5 and less than 10, and in particular is of the order of 3.

[0030] Advantageously and according to the invention, the growth reactor(30) is fed at a flow rate of carbon-containing precursor(s) of between5% and 80%, in particular of the order of 25%, of the total gas flowrate.

[0031] Advantageously and according to the invention the fluidised bedis heated to a temperature between 600° C. and 800° C.

[0032] The invention also covers a catalytic granular compositionsuitable for the implementation of a production process according to theinvention.

[0033] The invention thus relates to a catalytic granular compositioncomprising metallic particles containing at least one transition metalcarried by granules of solid support, so-called catalyst granules,characterised in that:

[0034] the catalyst granules are capable of being able to form afluidised bed,

[0035] the proportion by weight of metallic particles is between 1% and5%,

[0036] the metallic particles have a mean particle dimension of between1 nm and 10 nm as measured after heating at 750° C.

[0037] Throughout the text the expression “mean dimension” of theparticles or granules denotes the mean value (maximum of thedistribution curve of the dimensions of the particles or granules) ofthe dimensions of all the particle or granules as determined byconventional granulometry, in particular by the sedimentation rate,before use. The term “dimension” used in isolation denotes, for a givenparticle or a given granule, its largest real dimension as determinedfor example by static measurements obtained by observations with ascanning or transmission electron microscope, also before use.

[0038] As regards the metallic particles, the values of the dimension orof the mean dimension that are given throughout the text are thosemeasured before use for the production of the nanotubes, but afterheating the catalytic composition to 750° C. The inventors have in factfound that the dimensions of the particles before heating are not, ingeneral, capable of analysis, the particles being invisible under amicroscope. This operation is effected by contact with a neutralatmosphere, for example helium and/or nitrogen, at 750° C., for asufficient time in order to obtain stable values of dimensions. Thistime is in practice very low (of the order of a minute or a fewminutes). The activation may be effected in a fluidised bed (in thefluidised bed of the catalyst granules before feeding the carbon source)or in any other way, for example in a fixed bed. Furthermore thetemperature of 750° C. should be regarded solely as a value for themeasurement of the size of the particles and does not correspond to atemperature value that should necessarily be used in a process accordingto the invention or in order to obtain a catalytic composition accordingto the invention (even if this value may advantageously be that used incertain embodiments of the invention). In other words, it enables theinvention to be characterised uniquely by dimensional criteria, althougha catalytic composition not subjected to this specific temperature mayalso be in accordance with the invention.

[0039] Advantageously the catalytic granular composition according tothe invention is characterised in that the mean dimension of themetallic particles is between 2 nm and 8 nm, in particular of the orderof 4 to 5 nm, and in that for at least 97% by number of the metallicparticles, the difference between their dimension and the mean dimensionof the metallic particles is less than or equal to 5 nm, and inparticular is of the order of 3 nm.

[0040] The catalytic granular composition may comprise a small amount ofmetallic particles of dimension very much greater than the meandimension (typically more than 200% of the mean dimension).Nevertheless, advantageously and according to the invention thedimension of the metallic particles is less than 50 nm as measuredbefore use and installation in the fluidised bed, and after activationat 750° C.

[0041] Advantageously and according to the invention, the metallicparticles consist in an amount of at least 98 wt. % of at least onetransition metal and are substantially free of non-metallic elementsother than traces of carbon and/or oxygen and/or hydrogen and/ornitrogen. Several different transition metal may be used in order to bedeposited on the support granules. Likewise, several different catalyticcompositions according to the invention (whose support granules and/ormetallic particles have distinct characteristics) may be used as amixture. The traces of impurity may derive from the preparation processof the metallic particles. Apart from these traces, the 2% maximumremaining amount may comprise one or more metallic elements other than atransition metal. Preferably, advantageously and according to theinvention, the metallic particles consist of a pure metallic deposit ofat least one transition metal, with the exception of traces of impurity.Advantageously and according to the invention, the proportion by weightof metallic particles, in particular of iron, is between 1.5% and 4%.

[0042] Advantageously and according to the invention, the catalystgranules have a mean dimension between 10μ and 1000μ. Advantageously andaccording to the invention, the difference between the dimension of thecatalyst granules and the mean dimension of the catalyst granules isless than 50% of the value of the said mean dimension.

[0043] It has been found in fact that these dimensional distributions ofthe metallic particles and of the granules enable excellent results tobe obtained within the context of a fluidised bed.

[0044] Furthermore, advantageously and according to the invention, thesupport has a specific surface greater than 10 m²/g.

[0045] Advantageously and according to the invention, the support is aporous material having a mean pore size greater than the mean dimensionof the metallic particles. Advantageously and according to theinvention, the support is a mesoporous material, the pores having a meansize of less than 50 nm. Advantageously and according to the invention,the support is chosen from alumina (Al₂O₃), an activated carbon, silica,a silicate, magnesia (MgO), titanium dioxide (TiO₂), zirconia (ZrO₂), azeolite or a mixture of granules of several of these materials.

[0046] In particular, in the case where the carbon source is ethylene,advantageously and according to the invention the metallic particlesconsist of pure iron deposited in the dispersed state on granules ofalumina.

[0047] Advantageously, in a process for the production of nanotubesaccording to the invention, the catalyst granules are producedbeforehand by chemical deposition in the vapour phase of the metallicparticles on the support granules in a fluidised bed of the supportgranules fed with at least one precursor capable of forming the metallicparticles.

[0048] The invention also covers a process for the preparation of acatalytic granular composition according to the invention.

[0049] The invention thus relates to a process for the preparation of acatalytic granular composition comprising metallic particles containingat least one transition metal carried on solid support granules,so-called catalyst granules, in which a chemical deposition in thevapour phase of the metallic particles on the support granules isperformed in the vapour phase, characterised in that the deposition,particularly in the form of a chemical deposition, of the metallicparticles on the support granules is carried out in a fluidised bed ofthe support granules fed with at least one precursor capable of formingthe said metallic particles, and in that the support granules are chosenand the parameters of the deposition are adjusted so that:

[0050] the catalyst granules are capable of being able to form afluidised bed,

[0051] the proportion by weight of the metallic particles is between 1%and 5%,

[0052] the metallic particles have a mean particle dimension between 1nm and 10 nm as measured after heating to 750° C.

[0053] Advantageously and according to the invention the deposition iscarried out at a temperature between 200° C. and 300° C.

[0054] Advantageously and according to the invention the fluidised bedof the support granules is fed with at least one organometallicprecursor, in particular Fe(CO)₅.

[0055] Advantageously and according to the invention, the precursor(s)in the vapour state is/are continuously diluted in a gaseous mixturethat is supplied continuously to a deposition reactor under conditionscapable of ensuring the fluidisation of the support granules. Thus,advantageously and according to the invention, the fluidised bed is fedcontinuously with precursor(s). Advantageously and according to theinvention, the gaseous mixture comprises a neutral gas and at least onereactive gas. Advantageously and according to the invention, steam(water vapour) is used as reactive gas. Between 200° C. and 300° C. thesteam in fact enables the precursor Fe(CO)₅ to be decomposed, releasingatoms of Fe. In addition all manifestations of fritting andagglomeration of the metallic catalyst into excessively large metallicparticles is avoided.

[0056] The invention also relates to a process for the production ofnanotubes, a catalytic granular composition and a process for thepreparation of a catalytic granular composition, characterised by acombination of all or some of the characteristics mentioned hereinbelowor hereinafter.

[0057] Other objects, advantages and characteristics of the inventionare disclosed in the following description and examples, which refer tothe accompanying drawings in which:

[0058]FIG. 1 is a diagram of a first variant of an installation forimplementing a process for producing nanotubes according to theinvention,

[0059]FIG. 2 is a diagram of a second variant of an installation of aprocess for producing nanotubes according to the invention,

[0060]FIG. 3 is a histogram of the dimensions of the metallic particlesof a catalytic composition according to the invention obtained inExample 5,

[0061]FIGS. 4 and 5 are micrographs of the nanotubes obtained accordingto the invention as described in Example 9.

[0062]FIG. 1 is a diagram of an installation enabling a process forproducing nanotubes according to the invention to be implemented. Thisinstallation comprises two reactors: a reactor, so-called depositionreactor 20, for the synthesis of the catalyst, and a reactor, so-calledgrowth reactor 30, for the preparation of the nanotubes.

[0063] The deposition reactor 20 for the synthesis of the catalyst bychemical deposition in the vapour phase (CVD) comprises a glasssublimator 1 to which is added the organometallic precursor. Thissublimator comprises a fritted plate and may be heated to the desiredtemperature by a heating bath 2.

[0064] The neutral carrier gas 3, for example helium, which entrains thevapours of the organometallic precursor that is used is stored in acylinder and introduced into the sublimator 1 with the aid of a flowregulator (not shown).

[0065] The sublimator 1 is connected to a lower glass compartment 4 thatcomprises a fritted plate into which is introduced steam that serves toactivate the decomposition of the organometallic precursor. The presenceof steam enables a very active catalyst to be obtained. This compartment4 comprises a double jacket that is thermostatically controlled at atemperature that may be adjusted by means of a temperature regulator(not shown). The steam is entrained by a neutral carrier gas 5, forexample nitrogen, stored in a cylinder and added to the compartment 4with the aid of a flow regulator (not shown). A feed of neutral carriergas 6, for example nitrogen, is intended to adjust the flow rates tothose prevailing under the fluidisation conditions. This carrier gas 6is stored in a cylinder and added to the compartment 4 by means of aflow regulator (not shown).

[0066] The upper part of the compartment 4 is connected in a gas-tightmanner to a glass fluidisation column 7 of 5 cm diameter, which isequipped at its base with a gas distributor. This double jacket column 7is thermostatically controlled to a temperature that may be adjusted bymeans of a temperature regulator 8.

[0067] The upper part of the column 7 is connected to a vacuum pump 9via a trap in order to retain the released decomposition gases.

[0068] The procedure for implementing the examples relating to thepreparation of the catalysts by CVD is as follows: A mass Ma ofprecursor is added to the sublimator 1.

[0069] A mass Ms of support granules Ms is added to the column 7 and amass Me of water is added to the compartment 4 by means of a syringe. Avacuum is applied to the arrangement consisting of the compartment 4 andthe column 7. The temperature of the bed is raised to T1.

[0070] The sublimator 1 is heated to the temperature Ts and the pressureis fixed at the value Pa in the whole apparatus by introducing carriergases 3, 5 and 6 (total flow rate Q). The deposition then starts andlasts for a time t_(c).

[0071] At the end of the deposition, the temperature is restored toambient temperature by slow cooling and the vacuum pump 9 is switchedoff. Once the system has returned to ambient temperature and atmosphericpressure, the catalytic granular composition is removed from the column7 under an inert gas atmosphere (for example nitrogen); it is then readyto be used for the production of the nanotubes.

[0072] Two variants of the growth reactor 30, of different diameters,were used in the examples for growing the nanotubes.

[0073] In the first variant, shown in FIG. 1, the growth reactor 30consists of a quartz fluidisation column (2.5 cm diameter) 10 equippedin its middle part with a distributing plate (quartz fritted plate) 11on which is placed the powder of the catalytic granular composition. Thecolumn 10 may be heated to the desired temperature by means of anexternal heater 12 that can slide vertically along the fluidisationcolumn 10. In the procedure that is employed this heater 12 is arrangedeither at an upper position, where it does not heat the fluidised bed,or at a lower position, where it heats the bed. The gases 13 (neutralgas such as helium, carbon source, and hydrogen) are stored in cylindersand are added to the fluidisation column by means of flow regulators 14.

[0074] In the upper part, the fluidisation column 10 is connected in agas-tight manner to a trap 15 intended to collect any fine particles ofcatalytic granular composition or a mixture of catalytic granularcomposition and nanotubes.

[0075] The height of the column 10 is adapted so as to contain, duringoperation, the fluidised bed of the catalyst granules. In particular,the height is at least equal to 10 to 20 times the initial height of thebed of catalyst granules measured in the absence of the gaseous feed,and should correspond to the heated zone. In the examples a column 10 to70 cm in total height is chosen, which is heated over 60 cm of itsheight by the heater 12.

[0076] In the second variant (not shown) the growth reactor consists ofa stainless steel fluidisation column (5 cm diameter and 1 m totalheight, heated over the whole height) provided at its base with adistributor plate (stainless steel) on which is placed the catalystpowder. The column may be heated to the desired temperature by means oftwo fixed heaters and the said desired temperature is controlled by athermocouple dipping into the fluidised bed. The gases (neutral gas,carbon source and hydrogen) are stored in cylinders and are fed to thefluidisation column by means of flow regulators.

[0077]FIG. 2 shows a variant of a process according to the invention inwhich the catalytic granular composition is prepared, according to theinvention, continuously in the deposition reactor 20, removedcontinuously from this deposition reactor 20 through a line 25 a viawhich it is introduced to an intermediate buffer reservoir 26, fromwhich it is fed continuously, through a line 25 b, to the growth reactor30 where the nanotubes are produced. The deposition reactor 20 is fedcontinuously with support granules through a line 19 from a reservoir18. The powder of catalyst granules on which the nanotubes are attachedis removed continuously from the growth reactor 30 through an extractionline 27 that terminates in a buffer reservoir 28. The nanotubes may thenbe separated from the support granules and metallic particles in a knownmanner, following which they are stored in a storage reservoir 29.

[0078] In the variants shown in the Figures, a growth reactor 30different from the deposition reactor 20 is employed. By way ofvariation (not shown), the deposition reactor 20 may then be used forgrowing the nanotubes in a subsequent stage. However, this lattervariant means that the two stages have to be successively carried outwith different operating parameters, and there is a risk of interferencein the growth reaction, particularly in its initial phase, due toresidual byproducts from the deposition phase.

[0079] The procedure for implementing the examples relating to theproduction of nanotubes according to the invention is as follows:

[0080] a mass Mc of catalyst (catalytic granular composition accordingto the invention) is added to the fluidisation column 10 under an inertgas atmosphere.

[0081] With the heater 12 in its low position with respect to thecatalytic bed, its temperature is raised to the desired value Tn for thesynthesis of the nanotubes, either under an inert gas atmosphere orunder an atmosphere of a mixture of inert gas and hydrogen (reactivegas).

[0082] When this temperature is reached, the carbon source, the hydrogenand a neutral gas supplement are added to the column 10. The overallflow rate QT ensures a bubbling regime of the bed at the temperatureT_(n), without any leakage.

[0083] The growth of the nanotubes then starts and lasts for a timet_(n).

[0084] At the end of the growth stage the heater 12 is placed in thehigh position with respect to the catalytic bed, the gas flow ratescorresponding to the carbon source and hydrogen are stopped, and thetemperature is slowly restored to ambient temperature.

[0085] The procedure is similar in the case of reactors with fixedheaters.

[0086] The carbon nanotubes associated with the metallic particles andfixed to the support granules are removed from the growth reactor 30 andstored without any particular precautions. The carbon nanotubes may thenbe separated from the metallic particles and support granules so thatthey can be obtained in the pure state, for example by dissolution withacid as described in WO 01/94260.

[0087] The amount of carbon deposited is measured by weighing and bygravimetric thermal analysis.

[0088] The nanotubes produced in this way are analysed by transmissionelectron microscopy (TEM) and scanning electron microscopy (SEN) for thesize and dispersion measurements, and by X-ray crystallography and Ramanspectroscopy in order to evaluate the crystallinity of the nanotubes.

EXAMPLES

[0089] Preparation of the Catalysts

Comparison Example 1

[0090] A catalyst containing 2.6% Fe/Al₂O₃ is prepared by a known methodof liquid impregnation of metallic salts. The iron precursor is hydratediron nitrate Fe(NO₃)₃,9H₂O. The support granules of alumina have a meangrain size of 120μ, a density of 1.19 g/cm³ and a specific surface of155 m²/g. The carrier gas is nitrogen. The implementation of thepreparation of the catalyst is as follows:

[0091] The support is a mesoporous alumina. 100 g of this support aredehydrated in vacuo for 120 minutes. The appropriate amount of salt inorder to obtain 2.6% Fe/Al₂O₃ is contacted with the alumina in 250 cm³of deaerated ethanol. After 3 hours' contact time, the solvent isevaporated and the catalyst is dried overnight under reduced pressure(0.1 Torr). The catalyst is then calcined at 500° C. for 2 hours,following which it is reduced by a mixture of nitrogen and hydrogen(80/20 by volume) for 2 hours at 650° C.

[0092] The product obtained has a mean dimension of the metallicparticles equal to 13 nm and the variation of the dimensions of themetallic particles with respect to this value is, for at least 98% ofthe particles, at most of the order of 11 nm.

Example 2

[0093] A catalyst containing 2.6% Fe/Al₂O₃ is prepared in accordancewith the process according to the invention, in the deposition reactor20, as described hereinbefore but without using water to activate thedecomposition of the precursor. The organometallic precursor used is thecomplex Fe(CO)₅, while the support granules and the carrier gas that areused are the same as in Example 1. The various parameters are adjustedas follows:

[0094] Ma=9.11 g,

[0095] Ms=100 g,

[0096] Tl=220° C.,

[0097] Pa=22 Torr,

[0098] Ts=35° C.,

[0099] Q=82 cm³/min,

[0100] t_(c)=15 min.

[0101] The product obtained (catalytic granular composition according tothe invention) comprises metallic particles deposited on the granules.The dimension of the metallic particles after heating under nitrogen at750° C. for 5 minutes is equal to 4 nm, and the variation of thedimensions of the metallic particles with respect to this value is, forat least 97% of the particles, at most of the order of 3.5 nm.

Example 3

[0102] A catalyst containing 1.3% of Fe/Al₂O₃ is prepared according tothe invention. The carrier gas is nitrogen. The organometallicprecursor, the support granules and the carrier gas used are the same asin Example 2. The various parameters are adjusted as follows:

[0103] Ma=7.12 g,

[0104] Ms=150 g,

[0105] Me=10 g,

[0106] Tl=220° C.,

[0107] Pa=26 Torr,

[0108] Ts=35° C.,

[0109] Q=82 cm³/min,

[0110] t_(c)=7 min.

[0111] The product obtained has a mean dimension of the particles equalto 3 nm and the variation of the dimensions of the metallic particleswith respect to this value is, for at least 98% of the particles, atmost of the order of 2.5 nm.

Example 4

[0112] This example relates to the preparation of a catalyst containing2.5% Fe/Al₂O₃. The organometallic precursor, the support granules andthe carrier gas used are the same as in Example 2. The variousparameters are adjusted as follows:

[0113] Ma=17.95 g,

[0114] Ms=200 g,

[0115] Me=25 g,

[0116] Tl=220° C.,

[0117] Pa=20 Torr,

[0118] Ts=35° C.,

[0119] Q=82 cm³/min,

[0120] t_(c)=18 min.

[0121] The product obtained has a mean dimension of the metallicparticles equal to 4 nm and the variation of the dimensions of themetallic particles with respect to this value is, for at least 98% ofthe particles, at most of the order of 3.5 nm.

Example 5

[0122] This example relates to the preparation of a catalyst containing3.5% Fe/Al₂O₃. The organometallic precursor, the support granules andthe carrier gas used are the same as in Example 2. The variousparameters are adjusted as follows:

[0123] Ma=12.27 g,

[0124] Ms=100 g,

[0125] Me=25 g,

[0126] Tl=220° C.,

[0127] Pa=24 Torr,

[0128] Ts=35° C.,

[0129] Q=82 cm³/min,

[0130] t_(c)=20 min.

[0131] The product obtained has a mean dimension of the particles equalto 5 nm and the variation of the dimensions of the metallic particleswith respect to this value is, for at least 98% of the particles, atmost of the order of 4.5 nm. A histogram of the sizes of particles isgiven in FIG. 3.

[0132] In this figure the mean dimension of the particles is plottedalong the x axis and their number is plotted along the y axis.

Example 6

[0133] This example relates to the preparation of a catalyst containing5.65% Fe/Al₂O₃. The organometallic precursor, the support granules andthe carrier gas used are the same as in Example 2. The variousparameters are adjusted as follows:

[0134] Ma=9.89 g,

[0135] Ms=100 g,

[0136] Me=15 g,

[0137] Tl=220° C.,

[0138] Pa=23 Torr,

[0139] Ts=35° C.,

[0140] Q=82 cm³/min,

[0141] t_(c)=23 min.

[0142] The product obtained has a mean dimension of the particles equalto 6 nm and the variation of the dimensions of the metallic particleswith respect to this value is, for at least 98% of the particles, atmost of the order of 5.5 nm.

[0143] The results of Examples 1 to 6 are summarised in the followingTable I. TABLE I Size of the Metallic Example Precursor Method % FeParticles (nm) 1 Fe(NO₃)₃, 9H₂O impregnation 2.6 13 ± 11   2 Fe(CO)₅ CVD* 2.6 4.5 ± 4    3 Fe(CO)₅ CVD 1.3 3 ± 2.5 4 Fe(CO)₅ CVD 2.5 4 ± 3.55 Fe(CO)₅ CVD 3.50 5 ± 4.5 6 Fe(CO)₅ CVD 5.65 6 ± 5.5

[0144] Production of the Nanotubes

Comparison Example 7

[0145] Multi-walled nanotubes are produced using the catalyst ofcomparison example 1 containing 2.6% Fe/Al₂O₃. In this test the amountof catalyst was intentionally reduced so as not to obtain large yieldsand, more particularly, so as to be better able to determine theinfluence of the method of preparation of the catalyst. The variousparameters are adjusted as follows:

[0146] Mc=5 g,

[0147] Tn=750° C.

[0148] Q_(T)=320 cm³/min,

[0149] Amount of carbon added=3 g,

[0150] t_(n)=60 min.

[0151] Under these conditions, the amount of carbon deposited is 0.16 g,which should be compared with the result obtained in test 5 of Example12 (same percentage of iron and identical conditions), namely 1.57 g.The height of the bed remains substantially the same, whereas it changesfrom about 1 cm to 8.7 cm in test 5 of Example 12. The SEM and TEManalyses show that the multi-walled nanotubes comprise only a part ofthe deposit and that the encapsulated particles are in this caseextremely numerous. Thus, only a catalyst composition according to theinvention permits the selective production of multi-walled nanotubes ofhomogeneous mean dimensions.

Example 8

[0152] Multi-walled nanotubes are produced using the catalyst of Example2 containing 2.6% Fe/Al₂O₃ prepared without using water to activate thedecomposition of the precursor. In this test, the amount of catalyst wasintentionally reduced so as not to obtain high yields, and morespecifically so as to be better able to determine the influence of theactivation of the catalyst by water. The various parameters are adjustedas follows:

[0153] Mc=5 g,

[0154] Tn=750° C.

[0155] Q_(T)=320 cm³/min,

[0156] Amount of carbon added=3 g,

[0157] t_(n)=60 min.

[0158] Under these conditions, the amount of carbon deposited is 0.88 g,which should be compared with the result obtained in test 5 of Example12 (same percentage of iron and identical conditions except for theaddition of water), namely 1.57 g.

[0159] The activation of the catalyst by water thus promotes a highyield of nanotubes.

[0160] The TEM and SEM analyses show that the multi-walled nanotubesconstitute the only product of the deposition reaction.

Example 9

[0161] Nanotubes are produced using the catalyst of Example 4 containing2.5% Fe/Al₂O₃ and ethylene, and using a stainless steel reactor of 5 cminternal diameter. Five tests were carried out under the same conditionsso as to verify the reproducibility of the results.

[0162] The various parameters are adjusted as follows:

[0163] Mc=100 g,

[0164] Tn=650° C.

[0165] Q_(T)=1200 cm³/min,

[0166] Amount of carbon added=30 g,

[0167] t_(n)=120 min.

[0168] Under these conditions, the amount of carbon deposited is 27±0.2g in all the tests carried out, i.e. a yield of 90% with respect to theadded carbon. The SEM and TEM analyses show that the multi-wallednanotubes constitute the only product of the reaction. The pyrolyticcarbon or the encapsulated metal particles are largely absent from thedeposit. TEM micrographs of the nanotubes obtained are shown in FIGS. 4and 5. In FIG. 4 the scale represented by the continuous line is 400 nm.In FIG. 5 the scale represented by the continuous line is 20 nm. Theexternal diameter of the nanotubes is 20±5 nm and their internaldiameter is 4±2 nm, which corresponds substantially to the meandimension of the metallic particles. The X-ray and Raman analyses of thenanotubes obtained show the good degree of graphitisation of the latter;this can also be seen in FIG. 5, where the planes of the graphite can beobserved.

Example 10

[0169] Nanotubes are produced using the catalyst of Example 4 containing2.5% Fe/Al₂O₃ and ethylene, and using a stainless steel reactor of 5 cminternal diameter.

[0170] The various parameters are adjusted as follows:

[0171] Mc=100 g,

[0172] Tn=650° C.

[0173] Q_(T)=1200 cm³/min,

[0174] Amount of carbon added=45 g,

[0175] t_(n)=180 min.

[0176] Under these conditions, the amount of carbon deposited is 44 g,i.e. a yield of 97% with respect to the added carbon. The SEM and TEManalyses show that the multi-walled nanotubes constitute the onlyproduct of the reaction.

Example 11

[0177] A series of tests was carried out in a reactor of 2.5 cm diameterso as to investigate the influence of the amount of metal on thepreparation of multi-walled nanotubes using the catalysts of Examples 3to 6 and a catalyst containing 0.5% of iron prepared in a similarmanner, and with ethylene as carbon source. In these tests the amount ofcatalyst was intentionally reduced so as not to obtain large yields, andspecifically so as to be able better to determine the influence of theamount of metal.

[0178] The various parameters are adjusted as follows:

[0179] Mc=5 g,

[0180] Tn=750° C.

[0181] Q_(T)=320 cm³/min,

[0182] Amount of carbon added=3 g,

[0183] t_(n)=60 min.

[0184] The tests 1 to 5 of this example are summarised in the followingTable II. TABLE II Height of Bed Deposited after Deposition Test % FeCarbon (g) (cm) TEM Observation 1 0.5 0.52 3.2 multi-walled nanotubes 11.3 1.13 4 multi-walled nanotubes 2 2.5 1.90 6.2 multi-walled nanotubes3 3.5 2.29 8.6 multi-walled nanotubes 4 5.65 1.37 3 nanotubes +particles of encapsulated iron

[0185] The TEM and SEM analyses show that the multi-walled nanotubesconstitute the only product or virtually the only product of thedeposition reaction. The pyrolytic carbon or the particles ofencapsulated metal are particularly absent in tests 1 to 5. In test 1,since the concentration of iron is low (0.5%) the yield is greatlyaffected. In test 5, since the concentration of iron is high the size ofthe iron particles is large and the formation of particles ofencapsulated iron can be seen.

Example 12

[0186] A series of tests was carried out in a reactor of 2.5 cm diameterso as to investigate the influence of the temperature on the preparationof multi-walled nanotubes using the catalyst of Example 4 containing2.5% Fe/Al₂O₃ and ethylene as carbon source. In these tests the amountof catalyst was intentionally reduced so as not to obtain high yields,and so as to be better able to determine the influence of thetemperature.

[0187] The various parameters are adjusted as follows:

[0188] Mc=5 g,

[0189] Tn=variable from 500 to 850° C.

[0190] Q_(T)=320 cm³/min,

[0191] Amount of carbon added=3 g,

[0192] t_(n)=60 min.

[0193] The tests 1 to 6 of this example are summarised in Table III.TABLE III Deposited Height of Bed Temp. Carbon after Deposition Test(°C.) (g) (cm) TEM Observation 1 500 0.05 1.9 multi-walled nanotubes 2600 1.05 4.4 multi-walled nanotubes 3 650 1.13 5.5 multi-wallednanotubes 4 700 1.29 4.7 multi-walled nanotubes 5 750 1.57 8.7multi-walled nanotubes 6 850 1.86 4.7 nanotubes + pyrolytic carbon +particles of encapsulated iron

[0194] The TEM and SEM analyses show that the multi-walled nanotubesconstitute the only product or virtually the only product of thedeposition reaction. The pyrolytic carbon or the particles ofencapsulated metal are particularly absent in tests 1 to 5. In test 1,the temperature is too low for the reaction to proceed properly. In test6, the temperature is too high and a thermal decomposition of theethylene leads to the formation of pyrolytic carbon.

Example 13

[0195] This example relates to the preparation of nanotubes using thecatalyst of Example 4 containing 2.5% Fe/Al₂O₃ and ethylene, and using astainless steel growth reactor of 5 cm internal diameter.

[0196] The various parameters are adjusted as follows:

[0197] Mc=100 g,

[0198] Tn=650° C.,

[0199] Q_(T)=1405 cm³/min,

[0200] Amount of carbon added=48.5 g,

[0201] t_(n)=120 min.

[0202] Under these conditions, the amount of carbon deposited is 46.2 g,i.e. a yield of 95% with respect to the added carbon. The TEM and SEManalyses show that the multi-walled nanotubes constitute the onlyproduct of the reaction.

Example 14

[0203] This example relates to the preparation of nanotubes using acatalyst containing 0.5% Fe/Al₂O₃ prepared according to the proceduredescribed in Example 4 and ethylene, and using a stainless steel growthreactor of 5 cm internal diameter.

[0204] The various parameters are adjusted as follows:

[0205] Mc=100 g,

[0206] Tn=650° C.,

[0207] Q_(T)=1405 cm³/min,

[0208] Amount of carbon added=48.5 g,

[0209] t_(n)=120 min.

[0210] Under these conditions, the amount of carbon deposited is 20.4 g,i.e. a yield of 42% with respect to the added carbon. The TEM and SEManalyses show that the multi-walled nanotubes constitute the onlyproduct of the reaction. This example confirms the poor performances ofthe catalyst containing 0.5% of iron.

Example 15

[0211] This example relates to the purification of nanotubes producedusing a catalyst containing 2.5% Fe/Al₂O₃ and ethylene, and using astainless steel growth reactor of 5 cm internal diameter according tothe procedure described in Example 9. The solid powder leaving thereactor is added to a 2 l capacity flask in the presence of 500 ml ofwater and 500 ml of 98% sulfuric acid.

[0212] The various parameters are adjusted as follows:

[0213] M (nanotube powder+catalyst)=75 g,

[0214] V(H₂O)=500 ml,

[0215] V(H₂SO₄, 98%)=500 ml,

[0216] T=140° C.,

[0217] t_(n)=120 min.

[0218] After dissolving the alumina for 2 hours with acid, the solutionis filtered, the nanotubes are washed several times with water and thendried in a stove. The dry product (thermogravimetric analysis) consistsof 97% by weight of carbon nanotubes and 3% of iron.

1. A process for the selective production of ordered carbon nanotubes bydecomposition of a source of carbon in the gaseous state in contact withat least one solid catalyst in the form of metallic particles comprisingat least one transition metal carried on granules of solid support,so-called catalyst granules, capable of being able to form a fluidisedbed, the metallic particles having a mean dimension between 1 nm and 10nm as measured after activation by heating to 750° C., in which afluidised bed of the catalyst granules is formed in a reactor, theso-called growth reactor (30), and the carbon source is addedcontinuously to the growth reactor (30) in contact with the catalystgranules under conditions capable of ensuring the fluidisation of thebed of catalyst granules, the decomposition reaction and the formationof nanotubes, wherein: the catalyst granules of each catalyst areproduced beforehand by deposition of metallic particles on supportgranules in a fluidised bed of the support granules formed in a reactor,the so-called deposition reactor (20), fed with at least one precursorcapable of forming the metallic particles, and so as to obtain catalystgranules comprising a proportion by weight of the metallic particles ofbetween 1% and 5%, the catalyst granules are then placed in the growthreactor (30) without contact with the external atmosphere, followed bythe formation of the fluidised bed of the catalyst granules and theformation of nanotubes in the growth reactor (30).
 2. A process asclaimed in claim 1, wherein the catalyst granules are produced having amean dimension of the metallic particles of between 2 nm and 8 nm, andin which, for at least 97% by number of the metallic particles, thedifference between their dimension and the mean dimension of themetallic particles is less than or equal to 5 nm.
 3. A process asclaimed in claim 1, wherein the catalyst granules are produced with amean dimension of the particles of the order of 4 nm to 5 nm, and inwhich, for at least 97% by number of the metallic particles, thedifference between their dimension and the mean dimension of themetallic particles is of the order of 3 nm.
 4. A process as claimed inclaim 1, wherein the catalyst granules are produced with a dimension ofthe metallic particles of less than 50 nm.
 5. A process as claimed inclaim 1, wherein the fluidised bed is situated in the growth reactor(30) at a temperature between 600° C. and 800° C.
 6. A process asclaimed in claim 1, wherein the metallic particles consist in an amountof at least 98% by weight of at least one transition metal and aresubstantially free of non-metallic elements apart from traces of carbonand/or oxygen and/or hydrogen and/or nitrogen.
 7. A process as claimedin claim 1, wherein the metallic particles consist of a pure metallicdeposit of at least one transition metal.
 8. A process as claimed inclaim 1, wherein the catalyst granules are produced with a meandimension between 10μ and 1000μ.
 9. A process as claimed in claim 1,wherein the difference between the dimension of the catalyst granulesand the mean dimension of the produced catalyst granules is less than50% of the value of the said mean dimension.
 10. A process as claimed inclaim 1, wherein the support has a specific surface greater than 10m²/g.
 11. A process as claimed in claim 1, wherein the support is aporous material having a mean pore size greater than the mean dimensionof the metallic particles.
 12. A process as claimed in claim 1, whereinthe support is chosen from alumina, an activated carbon, silica, asilicate, magnesia, titanium dioxide, zirconia, a zeolite or a mixtureof granules of several of these materials.
 13. A process as claimed inclaim 1, wherein the metallic particles consist of pure iron depositedin the dispersed state on alumina granules.
 14. A process as claimed inclaim 1, wherein the deposition reactor (20) and the growth reactor (30)are different.
 15. A process as claimed in claim 14, wherein thedeposition reactor (20) and the growth reactor (30) are joined by atleast one gas-tight line (25 a, 26, 25 b) and wherein the growth reactor(30) is fed with catalyst granules through this line (25).
 16. A processas claimed in claim 1, wherein the catalyst granules are produced bychemical deposition in the vapour phase of the metallic particles on thesupport granules in a fluidised bed of the support granules in thedeposition reactor (20).
 17. A process as claimed in claim 1, whereinthe deposition of the particles on the support granules is carried outat a temperature between 200° C. and 300° C.
 18. A process as claimed inclaim 1, wherein the fluidised bed of the support granules in thedeposition reactor (20) is fed with at least one organometallicprecursor.
 19. A process as claimed in claim 18, wherein Fe(CO)₅ is usedas organometallic precursor.
 20. A process as claimed in claim 1,wherein the precursor(s) is continuously diluted in the vapour phase ina gaseous mixture that is continuously fed to the deposition reactor(20) under conditions suitable for ensuring the fluidisation of thesupport granules.
 21. A process as claimed in claim 20, wherein thegaseous mixture comprises a neutral gas and at least one reactive gas.22. A process as claimed in claim 21, wherein steam is used as reactivegas.
 23. A process as claimed in claim 1, wherein the fluidised bed ofthe catalyst granules is formed in a cylindrical growth reactor (30) ofdiameter greater than 2 cm and having a wall height capable ofcontaining 10 to 20 times the volume of the initial, non-fluidised bedof the catalyst granules as measured in the absence of any gaseous feed.24. A process as claimed in claim 1, wherein a fluidised bed of thecatalyst granules is formed in the growth reactor (30) under a bubblingregime that is at least substantially free of leakage.
 25. A process asclaimed in claim 1, wherein in order to form the fluidised bed ofcatalyst granules in the growth reactor (30): a bed of catalyst granulesis formed in the bottom of the growth reactor (30), the growth reactor(30) is fed from underneath the bed of catalyst granules with at leastone gas whose velocity is greater than the minimum velocity offluidisation of the bed of catalyst granules and less than the minimumvelocity for the occurrence of a plunger-type régime.
 26. A process asclaimed in claim 1, wherein in order to form the fluidised bed of thecatalyst granules in the growth reactor (30), the growth reactor (30) isfed from underneath the catalyst granules with the carbon source in thegaseous state and with at least one neutral carrier gas.
 27. A processas claimed in claim 1, wherein the growth reactor is fed with at leastone carbon-containing precursor forming the carbon source, with at leastone reactive gas and with at least one neutral gas, which are mixedbefore being introduced into the growth reactor (30).
 28. A process asclaimed in claim 1, wherein the carbon source comprises at least onecarbon-containing precursor chosen from hydrocarbons.
 29. A process asclaimed in claim 1, wherein the growth reactor (30) is fed with hydrogenas reactive gas.
 30. A process as claimed in claim 27, wherein the molarratio of the reactive gas(es) to the carbon-containing precursor(s) isgreater than 0.5 and less than 10, and in particular is of the order of3.
 31. A process as claimed in claim 27, wherein the growth reactor (30)is fed at a flow rate of carbon-containing precursor(s) of between 5%and 80%, in particular of the order of 25%, of the overall gaseous flowrate.
 32. A process for the preparation of a catalytic granularcomposition comprising metallic particles containing at least onetransition metal carried on solid support granules, so-called catalystgranules, in which there is effected a chemical deposition in the vapourphase of the metallic particles on the support granules, wherein thedeposition of the metallic particles on the support granules is carriedout in a fluidised bed of the support granules fed with at least oneprecursor capable of forming the metallic particles, and wherein thesupport granules are chosen and the parameters of the deposition areadjusted so that: the catalyst granules are capable of being able toform a fluidised bed, the proportion by weight of the metallic particlesis between 1% and 5%, the metallic particles have a mean particledimension between 1 nm and 10 nm as measured after activation by heatingto 750° C.
 33. A process as claimed in claim 32, wherein the depositionis carried out in the form of a chemical deposition in the vapour phase.34. A process as claimed in claim 32, wherein the deposition is carriedout at a temperature between 200° C. and 300° C.
 35. A process asclaimed in claim 32, wherein the fluidised bed of the support granulesis fed with at least one organometallic precursor.
 36. A process asclaimed in claim 32, wherein Fe(CO)₅ is used as organometallicprecursor.
 37. A process as claimed in claim 32, wherein theprecursor(s) is continuously diluted in the vapour state in a gaseousmixture that is continuously fed to a deposition reactor (20) underconditions capable of ensuring the fluidisation of the support granules.38. A process as claimed in claim 37, wherein the gaseous mixturecomprises a neutral gas and at least one reactive gas.
 39. A process asclaimed in claim 38, wherein steam is used as reactive gas.